Automatic transmissions of the type addressed by this invention include several fluid operated torque transmitting devices, referred to herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling, such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios. Accordingly, shifts from a given speed ratio to a lower speed ratio are referred to as downshifts, while shifts from a given speed ratio to a higher speed ratio are referred to as upshifts.
Shifting from one speed ratio to another generally involves a transition or state change of two clutches. That is, one clutch is engaged (on-coming) while another clutch is disengaged (off-going). The control of this invention applies to a class of shifts involving two sets of state changes These shifts are commonly referred to as double transition shifts. If the state changes involved in double transition shifts are not properly coordinated, the speed ratio may initially change in an unintended direction or at an unintended rate, thereby degrading the shift quality.